Show missing entries/dictionaries for lessons

[didoesdigital]

As per https://github.com/didoesdigital/typey-type/issues/5

… suggest at the start of a lesson which entries you might be missing from your dictionary that you should add before starting. For example, the stories lessons like Aesop’s Fables use British English spellings for words such as “neighbour”, so at the start of “Avaricious and Envious” Typey Type could show that your dictionary is missing "neighbour" and suggest that you grab the "Australian English with extra strokes" dictionary or add the entry "TPHAEUB/A*U": "neighbour", to your dictionary (if there are only a few entries missing).

This would make it easier to add more lessons that depend on specific dictionaries whereas it is now tricky to add lessons that will frustrate people by not having dictionary entries in Plover's dictionary.